Chemistry Department Colloquium

"Photo-Induced Chemical Bond Formation In Fluid Solution and at Sensitized TIO2 Interfaces"

Presented by Gerald J. Meyer, John Hopkins University

Wednesday, September 15, 2010, 11:00 am — Hamilton Seminar Room, Bldg. 555

Photo-initiated reactions that form chemical bonds are important for the conversion and storage of solar energy. Iodide oxidation yields an I-I bond that is key to sensitizer regeneration in dye-sensitized solar cells. The detailed mechanism(s) by which iodide oxidation yields the I-I bonds present in I2-• and I3-remain speculative. Here direct evidence that metal-to-ligand charge transfer (MLCT) excited states can directly yield iodine atoms that subsequently react to form I-I chemical bonds, kinetic and thermodynamic data are summarized in the Eo = -0.82 RuII/+(bpz)2+
Scheme 1
Jablonski-type diagram shown in Scheme 1. The

Eo = +0.82
RuII(bpz-)+, I2

kcr = 2.1 ± 0.3 × 1010 M-1 s-1
compound [Ru(bpz)2(deeb)](PF6)2, where bpz is
- 2 I¬
RuII(bpz-)+, I3-+ I¬-
kI2 = 3.3 × 109 M-1 s-1
+ I2×
2,2-bipyrazine and deeb is 4,4’-(CO2Et)2-2,2’-bpy
= 1.75 s
V vs SCE
is abbreviated Ru(bpz)2+. Excited state electron
+ I--1
kI = 2.4 ± 0.2 × 1010 M-1 s
transfer to yield the iodine atom is favored by 430
Eo = +0.93 RuII(bpz-)+, I
mV. Reaction of the iodine atom with iodide to
+ I
ket = 6.5 ± 0.3 × 1010 M-1 s-1 RuII*/+(bpz-)2+*
make an I-I bond lowers the free energy stored by 110 mV. Charge recombination to yield ground state products, Ru+ + I2-•  RuII + 2I- is downhill (-Go = 1.64 eV) and occurs with a rate constant of 2.1 x 1010 M-1 s-1, almost ten times larger than the I2-• disproportionation rate constant. Unwanted recombination to I2-• has been proposed to lower the efficiency of dye-sensitized solar cells and this data shows that it can be a very fast reaction [1].

Rapid photo-induced electron injection into TiO2 and regeneration by a donor, D, such as iodide or phenothiazine, yields sensitizers in an environment distinctly different from that prior to light absorption. Spectroelectrochemical measurements indicate that the injected electron influences the absorption spectr

Hosted by: Dmitry Polyanskiy

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